As is well-known, an anaerobic treatment method is known as a method of cleaning organic discharged water such as sewage and industrial effluent. In the anaerobic treatment method, organic matter is decomposed by the action of anaerobic microorganisms. Biogas containing methane, carbon dioxide and hydrogen sulfide is formed as a byproduct. The biogas can be utilized as fuel in gas boilers, etc. after hydrogen sulfide in the gas is removed (desulfurized). However, it has become a problem that because the formed hydrogen sulfide remains dissolved in the liquid phase in an anaerobic treatment tank, anaerobically treated water releases a bad smell. In addition, hydrogen sulfide may inhibit the activity of anaerobic microorganisms, which causes a reduction in the efficiency of a drainage treatment.
As a conventional method of removing hydrogen sulfide dissolved in anaerobically treated water, a method which includes blowing air into an anaerobic treatment tank (for example, Jpn. Pat. Appln. KOKAI Publication No. 7-16594) is known. As a method of removing hydrogen sulfide dissolved in an anaerobic reaction tank, Jpn. Pat. Appln. KOKAI Publication No. 7-16593, for example, is known. This method is a method which includes circulating a part of the treated water in a closed balancing tank between a closed anaerobic reaction tank and the balancing tank, to remove hydrogen sulfide from fermentation gas in the gas phase in either the reaction tank or the balancing tank, and then aerating the liquid phase in either the reaction tank or the balancing tank.
As a desulfurization method, a biological desulfurization method is also known. In this biological desulfurization method, hydrogen sulfide is removed from biogas by oxidization by the action of sulfur-oxidizing bacteria. The microorganisms are fed a nutritive salt-containing water so that they grow. This water also has a role in absorbing hydrogen sulfide from biogas and discharging a product formed by oxidization of hydrogen sulfide. In biological desulfurization, air is usually fed as the oxygen necessary for oxidization.